Abstract

An inherent problem to the study of waveguides with strong propagation losses by Scattering–type Scanning Near field Optical Microscopy is the coherent optical background field which disrupts strongly the weak detected near-field signal. We present a technique of heterodyne detection allowing us to overcome this difficulty while amplifying the near field signal. As illustrated in the case of highly confined Silicon on Isolator (SOI) structure, this technique, besides the amplitude, provides the local phase variation of the guided field. The knowledge of the complex field cartography leads to the modal analysis of the propagating radiation.

Experimental setup. The s-SNOM is a combination of an Atomic Force Microscope (AFM) with a confocal optical microscope. The complex amplitude of the guided field is obtained by incorporating the s-SNOM into one arm of an heterodyne mach-zehnder interferometer (see text for details).

s-SNOM signal demodulated at vertical probe oscillation frequency. The studied sample is an ion exchanged integrated waveguide. (A) Scheme of s-SNOM detection. β is the guided wave vector, kd corresponds to the average wave vector of both collected fields EBg and Ep towards the microscope objective. (B) s-SNOM raw optical image. Tilted fringes appear due to a coherent adding between the field scattered by the probe and a background field scattered in the direction of detection.

(A) and (B), 2D Fast Fourier Transform of the complex optical field (raw data from Figure 6). (C) and (D), respectively filtered amplitude and phase of the guided field after radiation modes subtraction on the 2D FFT spectrum. (see text for details).